There is an urgent need for new HIV-1 therapies targeting different steps of the viral replicative cycle to combat the growing prevalence of multidrug-resistant viruses and to reduce treatment toxicities. The chemokine receptor CCR5 serves as a critical portal of HIV-1 entry by acting as a fusion coreceptor in conjunction with CD4, the primary receptor for HIV-1. CCR5 plays a central role in virus transmission and pathogenesis, and therefore represents an attractive target for new HIV-1 therapies. PRO 140 is a unique humanized CCR5 monoclonal antibody (mAb) that offers a novel therapeutic profile. Unlike small-molecule CCR5 antagonists under development, PRO 140 broadly and potently inhibits CCR5-mediated HIV-1 entry without blocking or otherwise dysregulating the natural activities of CCR5. In addition, PRO 140 has demonstrated favorable tolerability and pharmacokinetic profiles in an ongoing Phase la clinical trial in healthy volunteers. PRO 140 is clearly differentiated from small molecules in terms of its lack of CCR5 antagonism, nonoverlapping patterns of viral resistance, antiviral synergy, excellent tolerability profile, and potential for infrequent (e.g., monthly) dosing. PRO 140 may therefore define a unique CCR5 inhibitor subclass, an issue that we will explore in Project 1. The highly innovative nature of this therapeutic approach is further underscored by the fact that no CCR5 inhibitor and no mAb to any target have been approved for HIV-1 therapy. In Project 3, we will utilize novel phenotypic and genotypic technologies to examine viral tropism, susceptibility and resistance to PRO 140. Initial studies will explore the natural variation in PRO 140 susceptibility using a broad panel of viruses selected from the 6000-member ViroLogic library with an emphasis on potential key controlling parameters. A primary focus of this Project is to monitor coreceptor usage and PRO 140 susceptibility in patients treated with PRO 140 in the clinical trials of Project 2. In this capacity, we will identify eligible study patients who present with pure R5 viruses, and we will monitor study participants for any change in coreceptor usage or PRO 140 susceptibility. We will examine whether any resistant variants represent outgrowth of pre-existing minor species or are the result of sequence evolution in response to therapy. The variants will be analyzed in vitro to determine their molecular mechanisms and genetic determinants of resistance, patterns of cross-resistance to HIV-1 entry and enzyme inhibitors, and intrinsic fusogenicity. Our laboratory research will be carried out in collaboration with Dr. Moore (Project 1), and we will address a complementary set of issues using distinct research methodologies. Project 3 is essential to the conduct of the first-in-HIV clinical studies of Project 2 and to the overall IPCP goal of optimally translating PRO 140 from an innovative treatment concept into a promising new investigational agent via an integrated series of preclinical and clinical proof-of-concept studies.